The invention relates to a process for the recovery of starch from the cellular tissue of root crops such as potatoes, in which the starting material is first washed and then ground, the pulp thus obtained is freed of most of the juice by means of a centrifuge in a preliminary stage, and the remaining mixture consisting essentially of starch and vegetable cell tissue is separated, after the addition of wash water, into starch milk and an aqueous phase carrying the fibrous substances. This washing out of the fibrous substances takes place in a plurality of successive stages.
For decades sieves have been used for this separating and washing process. Centrifuges have not been commonly used hitherto for this process because, depending on the throughput of a centrifuge drum, either too much fibrous substance and coagulated protein get into the starch or too much starch is lost with the fibrous substance. A reason for this is that a larger particle of a specifically lighter substance may have the same settling speed as a smaller particle of a specifically heavier substance. Separation of such particles is impossible by means of a centrifuge. A second reason is that the solid particles of different density--the fibers and the starch--are not present entirely in the form of individual components that are free of one another. The tissue particles, which are produced in the grinding and have been subjected to a preliminary juice removal, to some extent still contain starch grains within them, so that they have a greater specific weight than the pure fiber material. The difference in density is so slight that the separation of the weighted fibers from the starch in a centrifugal separator is very difficult.
It is known that the settling speed of a suspended particle is linearly proportional to the difference in density between the solid and the liquid vehicle, and is proportional to the square of the particle diameter. The fact that it is also dependent upon the distance of the particle from the axis of rotation and upon the angular velocity of the drum and the viscosity of the liquid vehicle can here be disregarded. In the framework of the present invention only the difference in density and the particle size are important.
From the settling speed and the distance the particle must travel in settling, which depends on the design of the drum, one can know the time which a particle of a certain size in a given suspension takes to cover this distance. From this time, taken as the minimum time of stay of the particle in the drum, and the capacity of the drum, it is possible to calculate the maximum throughput.
If a fluid contains only particles of the same solid, but having a wide range of diameters, then at a certain throughput the coarser particles are separated out, down to the size which has sufficient time to cover the settling distance. The finer particles are entrained by the vehicle fluid and emerge from the drum together with the latter. If, for example, it was desired also to separate solid particles having a diameter that is smaller by one power of ten, it would be necessary, on account of the quadratic relationship between the settling speed and the particle diameter, to increase the time of stay of the mixture in the drum by a factor of 100, i.e., to reduce the throughput to 1%. Thus it is possible by varying the throughput to influence the degree of separation.
If a liquid contains two kinds of solid of different density but the same particle size distribution, the largest particles of the specifically heavier component will settle the most rapidly and can be obtained in pure form. However, the smaller particles of the specifically heavier component correspond to larger particles of the specifically lighter component which have the same settling speed and therefore such particles cannot be separated from one another in a centrifugal separator.
Depending on the throughput of the separator, this mixed portion may be taken out of the drum together with the coarser, specifically heavier solids, or together with the liquid vehicle. It may also be divided between the two discharges in any desired ratio.
With regard to the recovery of starch from the cell tissue of root crops it must be mentioned that the fibers and the starch grains suspended in the water do not have the same size distribution. Therefore in the centrifugal separation of the pulp coming from the preliminary stage, to which washing water has been added, the disadvantages described above occur, i.e. either too many fibers and coagulated protein get into the starch or too much starch is lost with the fibers.
On account of this apparently insolvable problem the use of sieves in the wash-out part of the starch recovery system has been continued. Sieves, of course, divide according to particle size alone, regardless of the density, the maximum grain size being determined by the mesh size. The mesh size of the first sieve is such that the largest starch grains are able to pass through and the coarser fibers are held back. Finer fibers and coagulated protein pass through the mesh openings and have to be separated from the starch by centrifugation. To the extent that the fibers passing through the mesh openings still contain starch, such separation becomes very difficult, for the reasons stated above.
In any case, starch is still contained in the coarse fibers held back by the first sieve. They are therefore subjected to a regrinding in a second grinder and they are washed with the addition of water on at least one additional sieve. No further grinding is performed.
Fine fibers and coagulated protein also get into the starch from the second sieve, and from the third if any, while on the other hand the coarser fibers retained by the last sieve may still contain starch, thereby reducing the yield.
Sieves also have the disadvantage that the meshes of the sieve gradually become clogged with coagulated protein, requiring frequent cleaning.
To forestall this disadvantage and increase the yield of starch, sieves with coarser meshes are usually used, but this is again disadvantageous in that more fibers and coagulated protein get into the starch and interfere with the refining of the starch milk in the refining part of the system.